Ultra-dense LED projector using thinned gallium nitride

ABSTRACT

A small projector uses an ultra-dense array of gallium nitride (GaN) LEDs. However, epitaxial growth of GaN typically produces a GaN region that is 5 um or thicker. To achieve high pixel density, the LEDs have small area, so the resulting LED structures are tall and skinny. This is undesirable because it makes further processing more difficult and has higher optical losses. As a result, it is beneficial to reduce the thickness of the GaN region. In one approach, a wafer with the GaN region on substrate is bonded to a backplane wafer containing LED driver circuits. The substrate is then separated from the GaN region, exposing a buffer layer of the GaN region. The GaN region is thinned and then patterned into individual LEDs. Typically, the buffer layer is removed entirely.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. application Ser. No.16/154,603, “Method of Manufacturing Ultra-Dense LED Projector usingThinned Gallium Nitride,” filed Oct. 8, 2018, now U.S. Pat. No.10,768,515; which application claims priority under 35 U.S.C. § 119(e)to U.S. Provisional Patent Application Ser. No. 62/597,680, “New DisplayProcess Flow,” filed Dec. 12, 2017. The subject matter of all of theforegoing is incorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

This disclosure relates generally to light emitting diode (LED) displaysand, more specifically, to an ultra-dense gallium nitride LED display,such as for use in a contact lens.

2. Description of Related Art

A conventional LED direct emission display uses discrete red, green, andblue emitting LEDs arranged in an addressable array of composite pixels.Such displays have a fairly large pixel spacing due to the use ofseparate LED dies. Displays of this type typically have resolutions ofup to 500 pixels per inch (composite white pixels/inch) and about a 25um (micron) pitch from one colored pixel to the neighboring color pixel.

In another approach, red, green, and blue emitting LEDs are combined ona single die. However, the practical minimum pixel pitch achievable byconventional monolithic LED display technology is about 5-10 um withpixels several microns in size. Such LED displays may be referred to asmicro-displays since each pixel is several square microns in area. Verysmall displays may require die sizes of 1 mm or less. Such an LEDdisplay constructed using conventional technology typically is limitedin resolution or composite white pixel count.

Accordingly, what is needed are better approaches to forming anultra-dense (and, therefore, correspondingly higher resolution) LEDdisplay.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure have other advantages and features whichwill be more readily apparent from the following detailed descriptionand the appended claims, when taken in conjunction with the examples inthe accompanying drawings, in which:

FIG. 1 shows a top down view of a frontplane for a femtoprojectordisplay, and a magnified view of a hexagonal LED array within thefrontplane.

FIG. 2 shows a schematic diagram of certain circuits on a backplane fora femtoprojector display.

FIGS. 3A-3N show a process for manufacturing the femtoprojector displayof FIGS. 1-2.

FIG. 4 shows a cross-sectional view of adjacent LED pillars from afrontplane for a femtoprojector display.

FIG. 5 shows a cross sectional view of an eye-mounted display containinga femtoprojector in a contact lens.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The figures and the following description relate to preferredembodiments by way of illustration only. It should be noted that fromthe following discussion, alternative embodiments of the structures andmethods disclosed herein will be readily recognized as viablealternatives that may be employed without departing from the principlesof what is claimed.

A “femtoprojector” is a small projector that projects images from adisplay contained inside a contact lens onto a user's retina. Thedisplay and associated optical system are small enough to fit inside acontact lens. To meet this size requirement while still achievingreasonable resolution, the pixel sizes in the image source typically aremuch smaller than in image sources for other applications. For example,a conventional LED direct emission display uses discrete red, green, andblue emitting LEDs with resolutions of up to 500 pixels per inch(composite white pixels/inch) and about a 25 um (micron) pitch from onecolored pixel to the neighboring color pixel. In contrast, a display fora femtoprojector preferably has pixel sizes of less than 1 um² inemitting area with a pixel pitch of 2 um or less, so that when thedisplayed image is projected onto the retina, the image resolution mayalso be commensurate with the density of light receptors in the retina.

Gallium nitride (GaN) is a common material system used to fabricateLEDs. In a typical fabrication process, GaN layers are deposited on asubstrate such as sapphire. From the substrate upwards, the GaN regionmay include a buffer region, an n-doped region, an active region (e.g.,InGaN quantum wells) and a p-doped region. The distance from thesubstrate to the active region (i.e., buffer region plus n-doped region)may be especially thick, for example 5 um or more, because the firstseveral microns of GaN that are directly adjacent to the substratetypically is of poor quality. This is sometimes referred to as a bufferregion. Accordingly, the GaN region is grown thicker so that thematerial farther away from the substrate has the desired quality. If thepixels have small area, as is the case in femtoprojectors, the resultingLED structures can be tall and skinny, for example 5 um tall pillarswith a 1 um wide active area. This is undesirable because light producedby the active area reflects many times off the sidewalls before leavingthe LED pillar, resulting in higher optical loss. High aspect ratiostructures may also create fabrication difficulties. It is moredifficult to etch deep, narrow trenches, and also more difficult to fillthem with metals and/or other materials.

As a result, it is beneficial to reduce the thickness of this GaNregion. In one approach, a wafer with the GaN region on substrate isbonded to a backplane wafer containing LED driver circuits. Thesubstrate is separated from the GaN region, exposing the buffer layer ofGaN which is made thinner first by chemical mechanical polishing andthen etching. Thinning removes the buffer layer and reduces the heightof the LED pillars, which in turn reduces optical loss. Furthermore, iftrenches are etched to separate the GaN region into individual LEDpillars, then thinning the GaN region reduces the aspect ratio of thetrenches, which makes the etching process less challenging.

In many embodiments, the femtoprojector display includes a frontplaneand a backplane. FIG. 1 is a diagram of the frontplane, which containsan LED array. FIG. 2 is a diagram of the backplane, which containsaddressing and drive circuitry. FIG. 3 provides details for an exampleprocess for manufacturing the femtoprojector display of FIGS. 1-2.

FIG. 1 shows a top down view of a frontplane 110 for a femtoprojectordisplay, and a magnified view of the hexagonal LED array within thefrontplane. In one application, the ultra-dense LED display using thisfrontplane 110 may be used as a projector in a contact lens to projectan image directly onto a wearer's retina. The LED array within thisfrontplane 110 is shown as having a hexagonal shape, but other shapesare possible. A die containing the frontplane may have a rectangularshape. The dimensions in the following descriptions are also examples.

The frontplane 110 includes a central pixel area 112, a dead space area114, and an n-ring area 116. The area 116 is a termination area toelectrically connect one contact of all the LEDs in the array to acommon electrical contact on the backplane. The diameter of thefrontplane components shown may be about 0.7 mm, and the diameter of thepixel area 112 may be about 0.5 mm. The width of each pixel is less than2 um and preferably about 0.6 um. In one embodiment, the displaycontains more than 400,000 pixels with variable sizes of pixels from aminimum of 0.6 um to a maximum of 2 um.

Also shown in FIG. 1 is an expanded view of a portion of the pixel area112, showing red pixels (R) 118, green pixels (G) 120, and blue pixels(B) 122. In one embodiment, the LEDs are GaN-based LEDs and the activelayers of the LEDs within the pixels output blue light. The red andgreen pixels are formed using a phosphor, quantum dots or othercolor-conversion mechanisms to down convert the blue pump light tolonger wavelengths. The gap 124 between pixels is less than 0.5 um andpreferably about 0.2 um to increase the density, fill-factor, andresolution of the display. The space 125 between the pixels is filledwith a reflective metal, such as aluminum.

The die may be rectangular, even though the display portion 110 ishexagonal. The die may also contain various silicon circuitry forprocessing image signals, powering the device, addressing the pixels,etc.

FIG. 2 shows a schematic diagram of certain circuits on a backplane 250for use with the femtoprojector frontplane 110 of FIG. 1. FIG. 2schematically illustrates one possible addressing technique used on thebackplane 250 for addressing a particular pixel by applying a voltage tothe associated contact for that pixel. The die may be about 0.5-1 mmwide.

Image signals may be transmitted to the backplane 250 using wireless orother means. In one embodiment, radio frequency signals (e.g., about 13MHz) are received by an antenna and processed by a receiver/processor260. Power for the backplane 250 may be received by the antenna viaresonant inductive coupling and converted to the appropriate voltage andpolarity by a power converter 262. The power signal and the imagesignals may be at different frequencies so that the signals can beseparated. The power converter 262 and receiver/data processor 260 maybe integrated into the backplane chip 250 or integrated into a separatepower/data chip with the data receiver/processor 260 and the powerconverter 262 electrically connected to the display backplane 250 byconductors. The small size allows the femtoprojector display to beencased in a contact lens. The image signals may include addressingsignals that are decoded by a column decoder 264 and a row decoder 266.Traces 268 in the device layer of the backplane 250 form an array ofpixel locations. Control voltages on a selected column line and row lineturn on a transistor for conducting current to the selected pixel. Thecolor brightness may be controlled by pulse width modulation, byamplitude modulation or by other means. Low power CMOS switches may beused to address pixels. The relative brightness of the red, green, andblue pixels in a single full color pixel determines the perceived colorfor that composite pixel.

In an example of the display being incorporated in a contact lens, thepower converter 262 and receiver/processor 110 may be separated from thebackplane 250 in a separate chip, and both chips may be separatelyencased in the contact lens. The power/data chip is located away fromthe pupil so as to not obstruct vision. Small wires connect metal padson the backplane 250 to metal pads on the power/data chip. A thin wireloop antenna is also connected to pads on the power/data chip andencased in the contact lens.

Fabrication processing preferably is performed on a wafer scale. FIGS.3A-3N show a process for manufacturing a femtoprojector display usingthe frontplane and backplane of FIGS. 1-2. FIG. 3A is a legend thatshows the cross hatch patterns used in this series of figures. Metalsand other conductive materials are represented by a diagonal cross-hatchpattern. TCO is a transparent conductive oxide, for example indium tinoxide, aluminum zinc oxide, indium zinc oxide, indium cadmium oxide, andcarbon nanotube layers. GaN is the gallium nitride region that includesthe active region of the LED. GaN is a direct bandgap III-Vsemiconductor material that is well-suited for making LEDs. In FIG. 3A,dielectric and silicon dioxide (SiO₂) are materials that can provideelectrical isolation, and the dielectric stack is used to providewavelength selectivity.

FIG. 3B shows a frontplane wafer 300 and a separate backplane wafer 350.The frontplane wafer 300 contains a GaN region 310 that is epitaxiallygrown on a substrate 302, which will be referred to as the frontplanesubstrate. Starting from the substrate, the GaN region 310 includes abuffer region (not labelled), an n-doped region 312, the active region314 (represented by the dashed line) and a p-doped region 316.Typically, the GaN region 310 may have a total thickness ofapproximately 4-6 um. The active region 314 typically is very thin, forexample 0.1 um or less if an InGaN multiple quantum well structure isused. The p-doped region 316 is also thin, perhaps 0.2 um and typicallyless than 0.5 um. The remainder is the n-doped region 312 and bufferregion, which is relatively thick. This is because some thickness isrequired to allow the GaN growth to reach a sufficient quality. The GaNdirectly adjacent to the substrate 302 is a buffer region of poorquality.

P-contact metal 320 provides electrical contact to the p-doped region316. In some designs, it also acts as a reflector for light generated bythe active region 314. Examples of substrate 302 include sapphire. Otherexamples include silicon and silicon carbide. The frontplane wafer 300in FIG. 3B is unpatterned. That is, the GaN region 310 has not yet beenpatterned into individual LEDs. This significantly reduces alignmentrequirements in attaching the frontplane wafer 300 to the backplanewafer 350.

The backplane wafer 350 contains LED driver circuits on a substrate 352.FIG. 3B does not show the actual LED driver circuits but shows copperpads 362,366 that are used to make electrical contact between the drivercircuits and the LED contacts. Copper pads 366 provide electricalconnection from the n-contacts of all of the LEDs to a common cathode onthe backplane. Copper pads 362 provide connection from the p-contactmetal 320 of each LED to the addressable driver circuitry for that LED.The fill 368 between copper pads 362,366 may be SiO₂ or SiN_(x), whichis used as an etch stop in subsequent processing steps. Typically, thebackplane wafer 350 is a processed CMOS on silicon wafer. FIG. 3B alsoshows some alignment marks 390.

The frontplane wafer 300 is attached to the backplane wafer 350,resulting in the structure of FIG. 3C. In the example shown, theattachment is performed by non-solder surface bonding (e.g. by surfacediffusion) between the p-contact metal 320 and the copper pads 362,366.A conductive bonding agent 330 is deposited on the p-contact metal 320of the frontplane wafer. Examples of bonding agents include aluminum,indium tin oxide, aluminum-doped zinc oxide and aluminum with a surfacecoating of silicon, germanium or titanium to prevent oxidizing of thealuminum. The bonding agent 330 bonds to the copper pads 362,366. Thismechanically attaches the two wafers to each other. It also provides anelectrical connection between the p-contact metal 320 and the copperpads 362,366. This bonding step requires only rough alignment becausethe GaN region 310 has not yet been patterned into individual pixels.The bonding agent 330 may be selected to be compatible with furthersilicon wafer processing, and the bonding process itself occurs attemperatures and pressures that do not affect the already processed CMOSstructures on the backplane.

In an alternative approach, both metal layers may be coated with abonding agent. In this example, both the p-contact metal 320 and thecopper pads 362,366 may be coated with a bonding agent and then bondedtogether.

In an alternative approach, the frontplane wafer 300 is coated with asolder, such as Sn or In, and attached to the backplane wafer 350 usingsolder bonding. The copper pads 362, 366 may also be coated with solderin this method.

After the two wafers 300,350 are bonded together, the frontplanesubstrate 302 is removed. Laser liftoff may be used to remove thesapphire substrate 302. Chemical approaches may also be used to removethe sapphire substrate 302. This exposes the buffer region of the GaN310. Removal of the sapphire substrate 302 can create substantial shock,so doing that step before patterning the GaN region 310 is helpful froma standpoint of mechanical stability.

As shown in FIG. 3D, a dielectric fill such as silicon dioxide 335 isdeposited to planarize the surface. Examples of other materials includeSiN, benzocyclobutene (BCB), and spin-on glass.

The GaN region 310 is thinned, resulting in the structure of FIG. 3E.Chemical mechanical polishing and/or a blanket dry etch may be used toreduce the thickness of the GaN region 310. Thinning removes most or allof the GaN buffer region.

As shown in FIG. 3F, the GaN region is patterned into individual LEDpillars 342R,G,B, which form the LEDs for red, green and blue colorpixels for the display. In one approach, a deep etch is performedthrough the GaN region 310 and conductive bonding layer 330. The etchstops at the surface of the SiN_(x)/SiO₂ region 368. This also exposesthe pads 366 for the common cathode.

The GaN region 310 of FIG. 3E is patterned into the individual LEDpillars 342 of FIG. 3F by etching trenches between the pillars. Thesetrenches may be 0.2-0.3 um wide. If the GaN were not thinned, the gapsbetween pillars would be 6 um tall and 0.2-0.3 um wide for aheight:width aspect ratio in the range of 20-30. It is difficult to etcha narrow trench with such a high aspect ratio. It is also difficult tofurther process narrow trenches, such as coating or filling them.Thinning the GaN region reduces the aspect ratio of the trenches, whichmakes etching and other processes easier.

Thinning the GaN region also reduces the aspect ratio of the LED pillars342, which improves their optical performance. FIG. 3 is not drawn toscale. Rather, the figures are drawn to illustrate the order of processsteps and spatial relationships between various material layers. The LEDpillars 342 without thinning may be 4-6 um tall and 0.5-1 um wide, for aheight:width aspect ratio in the range of 4-12. Thinning the GaN regionmay remove 2-4 um of material, reducing the LED pillar to a height ofnot more than 2 um and reducing the aspect ratio by a factor of 2× to3×.

Note that some areas of the bonding agent 330 are etched away when theGaN pixels are patterned. The conductive bonding agent 330 is selectedso that it does not coat (e.g. sputtered as an etch byproduct) thesidewalls of the GaN LED pillars 342 in order to prevent shorting of theLEDs.

In FIG. 3G, the sidewalls of the LED pillars 342 are passivated. In oneapproach, atomic layer deposition is used to deposit a passivationmaterial 344 over the entire structure, including on both the tops andsidewalls of the LED pillars 342. Example passivation materials includeAl₂O₃, TiO₂, SiO₂, SiN_(X), HfO_(X), and NbO_(X). A directional etch isused to etch the passivation material 344. This removes the passivationmaterial from the horizontal surfaces, including the tops of theindividual LED pillars, but leaves the passivation material on thesidewalls of the LED pillars. This approach does not require anylithography or fine alignment. The passivation material 344 electricallyinsulates the sides of the LED pillars 342 to prevent shorting of theLEDs.

The electrical connection between the LED pillars 342 and the commoncathode 366 are formed in FIGS. 3H-3I. In FIG. 3H, a thin, reflectivemetal lining 346 such as aluminum or ruthenium is deposited, followed bya fill metal 347 such as copper, aluminum or gold. This is planarizedvia chemical mechanical polishing to expose the GaN surface. In FIG. 3I,a thin layer of a transparent conductive oxide 348 (e.g. ITO) isdeposited as a current spreading layer that provides an electricalconnection to each pixel.

This completes the LED structures in the femtoprojector. Referring toFIG. 3I, the individual p-contacts for each LED are from the p-contactmetal 320 through the conductive bonding agent 330 to the copper pad 362to the addressable driver circuit. The n-contacts for all LEDs arethrough the transparent conductive oxide 348 to the metal 346/347 to thecopper pad 366 to the common cathode.

Note that this process uses only one lithographic step. In FIG. 3F, fineresolution lithography is used to align the LED pillars 342 with theircorresponding driver pads 362. However, insulating the sidewalls of thepillars (FIG. 3G) and forming the electrical network to the top contactof the individual LED pillars (FIGS. 3H-3I) are done without anylithography steps.

FIG. 4 shows a cross-sectional view of two adjacent LED pillars 342.FIG. 4 is drawn to scale using specific dimensions for the sake ofillustration, but LED displays can be constructed using otherdimensions. In this example, the LED pillars 342 include 2.75 um ofn-doped GaN 312, 0.05 um of InGaN multiple quantum well action region314, and 0.2 um of p-doped GaN 316. Light is generated at the activeregion 314, so the optical path from the active region to the exit ofthe LED at the far end of the n-doped region 312 is 2.75 um long. If theLED pillar is 1 um wide, then this optical path has a height:widthaspect ratio of 2.75:1. Light may reflect multiple times from thesidewalls before reaching the exit. Each reflection introduces someoptical loss.

As described above, the sides of the LED pillar 342 are coated with adielectric 344 (0.02 um), a metal layer 346 that acts as a bottomreflector (0.05 um) and metal fill 347 (0.16 um) which providesstructural support and may also provide electrical connection to thecommon cathode pads. In order to fabricate these structures, with a 3 umtall LED pillar, a 3 um tall and 0.3 um wide trench is first etchedbetween the LED pillars. This is a trench with a height:width aspectratio of 10:1. The trench is 3 um deep because it electrically isolatesthe p-doped GaN 316 from adjacent pixels. The trench also extendsthrough bottom metal 320 and bonding agent 330 so the total trench depthmay be more than 3 um. The sidewalls are then conformally coated withthe dielectric 344 and the reflector 346. This narrows the trench to0.16 um (but still 3 um tall), when it is filled with metal 347.

If the GaN region had not been thinned, the LED pillar 342 would havebeen even taller, say 6 um tall. At this height, the optical path fromactive region 314 to exit would have an aspect ratio of 5.75:1, morethan doubling the number of reflections before exiting the LED. Inaddition, the trench to be etched would have an aspect ratio of 20:1 andthe metal fill 347 would occupy a space with aspect ratio of almost40:1.

The dimensions given above are just examples. Typical ranges are thefollowing. For the gap between pillars: 0.2-1 um for the full gap width,0.01-0.05 um for dielectric 344 and 0.03-0.10 um for reflector 346. Forthe LED pillar: 1-5 um for n-GaN 312, 0.05±0.025 um for MQW 314, and0.2±0.1 um for p-GaN 316. Below the GaN (not shown in FIG. 4): <50 nmfor p-contact metal 320 (ITO, Ag, or NiAu) and <1 um for bonding agent330.

Returning to FIG. 3, the steps in FIGS. 3A-3I were described in thecontext of attaching an LED frontplane wafer 300 to a silicon CMOSbackplane wafer 350. This could also be done at the die level. Forexample, an LED frontplane wafer may be diced and individual, or groupsof, LED frontplane dies 300 attached to corresponding backplane dies350. The backplane dies may also be in wafer form or already diced intoindividual or groups of dies.

FIGS. 3J-3N show additional steps to add color conversion materials,which in this example are quantum dot materials. In FIG. 3J, amulti-layer dielectric stack 373 is deposited on the top surface. Thedielectric stack 373 provides wavelength selectivity. For example, ifthe LEDs produce blue light, then the dielectric stack 373 may bedesigned to transmit blue light and reflect red and green light. Asacrificial layer 374, such as a thick oxide (e.g., SiO₂ or SiN_(X)) isdeposited on the dielectric stack 373. In FIG. 3K, the dielectric stack373 and thick oxide 374 are patterned into pillars 372R,G,B aligned withthe LED pillars 342R,G,B. In FIG. 3L, a thin, reflective metal lining375 such as aluminum or ruthenium is deposited, followed by a fill metal376 such as copper or aluminum. In FIG. 3M, this is planarized viachemical mechanical polishing to expose the oxide 374. In FIG. 3N, thesacrificial layer 374 is removed, for example using a wet or drychemical etch. This leaves a space 377 into which color conversionmaterial may be deposited. A typical height for the space 377 is in arange of 1-3 um. Different materials may be deposited into differentspaces 377, for example a quantum dot material 377R for color conversionfrom blue to red, a quantum dot material 377G for color conversion fromblue to green, and no material 377B for the blue pixel. Anotheralternative is light scattering particles (e.g. TiO₂) for the bluepixel.

Alternatively, the color conversion layers may be formed as a separatedie and then attached to the LED die on top of the backplane. This isthen singulated to form separate femtoprojector displays, typically withwidth less than 1 mm.

One possible use of such a monolithic ultra-dense LED display is toembed the display in a contact lens so that the displayed image overlays(or replaces) the wearer's view of the real world. FIG. 5 shows a crosssectional view of an eye-mounted display containing a femtoprojector 500in a contact lens 550.

FIG. 5 shows an embodiment using a scleral contact lens which ispreferred because scleral lenses are designed to not move on the cornea,but the contact lens does not have to be scleral. The aqueous of theeyeball is located between the cornea 574 and the crystalline lens 576of the eye. The vitreous fills most of the eyeball including the volumebetween the crystalline lens 576 and the retina 578. The iris 584 limitsthe aperture of the eye.

The contact lens 550 preferably has a thickness that is less than twomm, and the femtoprojector 500 preferably fits in a 2 mm by 2 mm by 2 mmor smaller volume. The contact lens 550 is comfortable to wear andmaintains eye health by permitting oxygen to reach the cornea 574. Thefemtoprojector 500 includes an image source 512/514 and an opticalsystem 530. The image source includes a backplane 512 and a frontplane514, examples of which have been described above. In this example, thebackplane 512 is a CMOS application specific integrated circuit (ASIC)and the frontplane 514 includes a GaN LED array. The backplaneelectronics 512 receive data packets from a source external to theeye-mounted display. The backplane ASIC 512 converts the data packets todrive currents for the frontplane GaN LED array 514, which produceslight that is projected by the optical system 530 to the user's retina578.

The array of light emitters 514 may have non-uniform resolution. Forexample, the central area of the array may be imaged onto the fovea andtherefore the center pixels have higher resolution (i.e., smaller pitchbetween pixels) compared to pixels on the periphery of the array. Thepitches of the frontplane 512 and backplane 514 may be matched, in whichcase there is less area for each pixel driver in the center of thebackplane compared to the periphery. Alternately, the backplane 514 mayhave a uniform pitch, where the frontplane 512 still has a variablepitch. In one approach, a wiring layer bridges between the uniform pitchbackplane 514 and variable pitch frontplane 512. By using differentwiring layers, the same backplane may be used with differentfrontplanes.

Eye-mounted femtoprojector displays may use a 200×200 array of colorpixels. The display may be monochromatic or color. A three-color displaywith three LEDs per color pixel may have a total of at least 120,000LEDs.

Another possible use of the monolithic ultra-dense LED display is ineyewear, such as glasses or goggles, to create an immersive visualexperience or an image that overlays the wearer's view of the realworld, such as in an augmented, mixed, or artificial realityapplication.

Although the detailed description contains many specifics, these shouldnot be construed as limiting the scope of the invention but merely asillustrating different examples. It should be appreciated that the scopeof the disclosure includes other embodiments not discussed in detailabove. Various other modifications, changes and variations which will beapparent to those skilled in the art may be made in the arrangement,operation and details of the method and apparatus disclosed hereinwithout departing from the spirit and scope as defined in the appendedclaims. Therefore, the scope of the invention should be determined bythe appended claims and their legal equivalents.

What is claimed is:
 1. A display device comprising: a backplane diecomprising an array of LED driver circuits on a backplane substrate; anda frontplane structure attached to the backplane die, the frontplanestructure comprising a thinned gallium nitride region patterned intoindividual LED pillars having an active region of width not more than 2um, wherein the gallium nitride region is thinned to reduce a height ofthe individual LED pillars as measured from the active region to a topof the gallium nitride region to less than 3 um, wherein the individualLED pillars form at least a 200×200 array of color pixels.
 2. Thedisplay device of claim 1 wherein the individual LED pillars produceblue light, and the display device further comprises: a first colorconversion material on top of a first subset of the individual LEDpillars, the first color conversion material converting the blue lightproduced by the first subset of individual LED pillars to green light;and a second color conversion material on top of a second subset of theindividual LED pillars, the second color conversion material convertingthe blue light produced by the second subset of individual LED pillarsto red light.
 3. The display device of claim 2 further comprising:scattering particles on top of a third subset of the individual LEDpillars.
 4. The display device of claim 1 wherein adjacent individualLED pillars are separated by trenches that have a width of not more than0.5 um.
 5. The display device of claim 4 wherein the trenches separatingadjacent individual LED pillars have a height-to-width aspect ratio ofat least 4:1.
 6. The display device of claim 1 wherein sidewalls of theindividual LED pillars are passivated, and the display device furthercomprises: electrical contacts to top contacts of the individual LEDpillars.
 7. A display device comprising: a backplane die comprising anarray of LED driver circuits on a backplane substrate; a frontplanestructure attached to the backplane die, the frontplane structurecomprising a thinned gallium nitride region patterned into individualLED pillars having an active region of width not more than 2 um, whereinthe gallium nitride region is thinned to reduce a height of theindividual LED pillars as measured from the active region to a top ofthe gallium nitride region to less than 3 um; and a multi-layerdielectric stack on top of at least some of the individual LED pillars,the multi-layer dielectric stack providing wavelength selectivity.
 8. Adisplay device comprising: a backplane die comprising an array of LEDdriver circuits on a backplane substrate; a frontplane structureattached to the backplane die, the frontplane structure comprising athinned gallium nitride region patterned into individual LED pillarshaving an active region of width not more than 2 um, wherein the galliumnitride region is thinned to reduce a height of the individual LEDpillars as measured from the active region to a top of the galliumnitride region to less than 3 um; and color conversion material on topof at least some of the individual LED pillars, the color conversionmaterial converting light produced by the individual LED pillars to adifferent wavelength.
 9. The display device of claim 8 wherein the colorconversion material comprises quantum dots.
 10. The display device ofclaim 8 wherein the color conversion material comprises individualpillars of color conversion material on top of the individual LEDpillars.
 11. A display device comprising: a backplane die comprising anarray of LED driver circuits on a backplane substrate; and a frontplanestructure attached to the backplane die, the frontplane structurecomprising a thinned gallium nitride region patterned into individualLED pillars having an active region of width not more than 2 um, whereinthe gallium nitride region is thinned to reduce a height of theindividual LED pillars as measured from the active region to a top ofthe gallium nitride region to less than 3 um; and wherein the displaydevice is sufficiently small to fit into a contact lens.
 12. A displaydevice comprising: a backplane die comprising an array of LED drivercircuits on a backplane substrate; and a frontplane structure attachedto the backplane die, the frontplane structure comprising a thinnedgallium nitride region patterned into individual LED pillars having anactive region of width not more than 2 um, wherein the gallium nitrideregion is thinned to reduce a height of the individual LED pillars asmeasured from the active region to a top of the gallium nitride regionto less than 3 um; and wherein thinning the gallium nitride regionreduces a ratio of the height to the width to less than 5:1.
 13. Adisplay device comprising: a backplane die comprising an array of LEDdriver circuits on a backplane substrate; a frontplane structureattached to the backplane die, the frontplane structure comprising athinned gallium nitride region patterned into individual LED pillarshaving an active region of width not more than 2 um, wherein the galliumnitride region is thinned to reduce a height of the individual LEDpillars as measured from the active region to a top of the galliumnitride region to less than 3 um; and a metal layer to metal layer bondattaching the frontplane die to the backplane die.
 14. The displaydevice of claim 13 wherein the bond uses a bonding agent selected fromaluminum; indium tin oxide; aluminum-doped zinc oxide; aluminum with asurface coating of silicon, germanium or titanium; Sn solder and Insolder.
 15. The display device of claim 13 wherein the backplanesubstrate is a silicon substrate, the LED driver circuits are CMOScircuits, and the metal layer on the backplane substrate is a copperlayer.
 16. An eye-mounted display comprising: a contact lens containinga femtoprojector display, the femtoprojector display comprising: abackplane die comprising an array of LED driver circuits on a backplanesubstrate, the backplane die receiving data and converting the data intodrive currents; and a frontplane structure attached to the backplanedie, the frontplane structure comprising a thinned gallium nitrideregion patterned into an array of individual LED pillars having anactive region of width not more than 2 um, wherein the gallium nitrideregion is thinned to reduce a height of the individual LED pillars asmeasured from the active region to a top of the gallium nitride regionto less than 3 um; the individual LED pillars driven by the drivecurrents to produce light; wherein the individual LED pillars form atleast a 200×200 array of color pixels; and an optical system thatprojects the light from the array of individual LED pillars onto auser's retina.
 17. The eye-mounted display of claim 16 wherein thefemtoprojector display is a color display, and the femtoprojectordisplay further comprises: color conversion material on top of at leastsome of the individual LED pillars, the color conversion materialconverting light produced by the individual LED pillars to a differentwavelength.
 18. A display system comprising: eyewear containing afemtoprojector display that is not larger than 2 mm×2 mm×2 mm, thefemtoprojector display comprising: a backplane die comprising an arrayof LED driver circuits on a backplane substrate, the backplane diereceiving data and converting the data into drive currents; and afrontplane structure attached to the backplane die, the frontplanestructure comprising a thinned gallium nitride region patterned into anarray of individual LED pillars having an active region of width notmore than 2 um, wherein the gallium nitride region is thinned to reducea height of the individual LED pillars as measured from the activeregion to a top of the gallium nitride region to less than 3 um; theindividual LED pillars driven by the drive currents to produce light;and an optical system that projects an image from the light from thearray of individual LED pillars.
 19. The display system of claim 18wherein the femtoprojector display is a color display, and thefemtoprojector display further comprises: color conversion material ontop of at least some of the individual LED pillars, the color conversionmaterial converting light produced by the individual LED pillars to adifferent wavelength.